1. Field of the Invention
The present invention relates to a spirofluorene-based polymer and an organic electroluminescent device using the same, and more particularly to a spirofluorene-based polymer containing a spirofluorene structure as a basic unit, which has a high charge transport capability and blue light-emitting property, and has thermal, optical and electrical stability due to introduced functional moieties substituted by fluorine having an electrical property into the ends of the polymer, and an organic electroluminescent device using an organic layer containing the spirofluorene-based polymer. The device has improved efficiency and color purity and reduced driving voltage.
2. Description of the Related Art
Organic electroluminescent display devices can use either small molecular weight compounds or polymer compounds, depending on their composition and manufacture. In the preparation of a device using small molecular weight compounds, a thin film is formed by a vacuum deposition. This has the advantages of easy purification and high purity of the luminescent materials, and easy embodiment of color pixels. However, for practical applications, there are needs for enhancement in quantum efficiency, prevention of crystallization of thin films, and enhancement in color purity.
Many studies have been made into using polymers, since the discovery of the electroluminescence of poly(1,4-phenylenevinylene)(PPV) which is a π-conjugated polymer. π-conjugated polymers have an alternating chemical structure of single bonds (or σ bonds) and double bonds (or π bonds), so that the polymers have delocalized π electrons capable of freely moving along with the polymer chain. The n-conjugated polymers have semi-conductive properties, and thus the whole visible light region corresponding to the highest occupied molecular orbitals-lowest occupied molecular orbitals (HOMO-LUMO) energy band-gap of polymers can be easily obtained through molecular design of the π-conjugated polymer, when the polymers are employed in an electroluminescent layer of an electroluminescent device. In addition, a thin film of polymer can simply be formed by spin coating or printing. Accordingly, the manufacturing process is very simple and cost-effective. Furthermore, the mechanical properties of a thin film of a π-conjugated polymer are excellent, due to its high glass transition temperature (Tg).
However, the device using polymer compounds has several problems, including low color purity, high driving voltage and low efficiency. Many studies have been made into overcoming these problems. For example, poly(9,9-dialkylfluorene)(PAF) or fluorene-containing polymers, as disclosed in U.S. Pat. No. 6,169,163 (entitled Fluorene-containing polymers and compounds useful in the preparation thereof to Woo, et al., and issued on Jan. 2, 2001), have a high light-emitting efficiency and may achieve high efficiency in color via polymerization. However, due to the electromers resulting from oxidation by light, a defect in the polymer backbone, and the interaction between molecules due to the remaining active ends of polymer compounds after polymerization, the lifetime of the materials is insufficient. Thus, the short lifetime of blue light-emitting polymer has inhibited the development of electroluminescent display devices using the polymer.
Recently, a method has been reported in which fluorene-containing polymers are end-capped with triarylamine derivatives having hole transport capability. [T. Miteva, A. Meisel, W. Knoll, H. G. Nothofer, Adv. Mater. 13(8), 565(2001)]. In this method, hole trapping occurs efficiently at the ends of the polymer, and thus the efficiency of the device may be improved. However, the lifetime stability of the device is not sufficient. Furthermore, a method of improving color purity was proposed in which a polymer containing fluorene and anthracene was end-capped with 2-bromofluorene and 2-bromo-9-fluorene to prevent the formation of excimers by the interactions between polymer molecules at its ends. (J. I. Lee, G. Klaerner and R. D. Miller, “Oxidative Stability and Its Effect on the Photoluminescence of Poly(Fluorene) Derivatives: End Group Effects,” Chem. Mater. 11, 1083 (1999)) However, this method could not overcome the problem that a position of C-9 was thermally unstable under the applied voltage, thus changing its color purity. Another method was reported in which a fluorene-containing copolymer was end-capped with a functional group capable only of chain extension or crosslinking, and not having electrical characteristics, such as enhancement of the performance of a device (U.S. Pat. No. 5,708,130 entitled 2,7-aryl-9-substituted fluorenes and 9-substituted fluorene oligomers and polymers to Woo, et al. and issued on Jan. 13, 1998).
Introduction of end cappers has been in most cases limited to fluorene-containing polymers, and the known end cappers have insufficient effect on improving lifetime. Thus, there is still a need for improvement in fluorene-containing polymers.